For the first question a lot of them can be solved by remembering some Matrix multiplication rules. ABC = (AB)C = A(BC). However AB does not equal BA.

For the second question remember the requirements for a subspace: Does the space contain the zero vector? If V and U are in the subspace then is V+U is also in the subspace? for some scalar r if U is in the subspace is rU in the subspace? If you answered yes to all of the previous questions then it is a subspace. If you answered no to any one of them it is not.